This invention relates to dewars, and, more particularly, to dewars used in measurement of small magnetic fields.
Biomagnetometers are devices that measure the small magnetic fields produced by a living organism. The biomagnetometer includes a number of sensors arranged in an array external to the organism, which measure the magnetic field at a number of locations. Each sensor has a magnetic field pickup coil. When a small magnetic flux change penetrates the pickup coil, a small electrical current flows in the coil. This small current is detected by a sensitive detector of electrical currents, preferably a Superconducting QUantum Interference Device, sometimes known by the acronym "SQUID". The output of the various SQUIDs, after signal conditioning and filtering, is provided to a computer which stores and analyzes the data.
The SQUIDs operate only at superconducting temperatures, and to attain the best system performance the pickup coils and SQUIDs are usually placed into a cryogenically cooled vessel termed a dewar. The dewar contains a reservoir of a cryogenic fluid such as liquid helium, with the sensors and SQUIDs in a structure termed a "dewar tail" that extends downwardly from the reservoir. A thermal shield surrounds the dewar and is supported from the neck at the upper end of the dewar. The thermal shield aids in maintaining the low temperature of the sensors and SOUIDs by intercepting radiant heat that flows inwardly through an outer dewar body and outer insulation. The heat received by the thermal shield is conducted upwardly through the thermal shield to the point of its attachment at the dewar neck, where the heat is transferred to the cryogenic gas that evaporates from the reservoir. The entire structure must be nonmagnetic in order to avoid interfering with the operation of the pickup coils and SOUIDs.
The construction of the thermal shield is a principal focus of the present invention. The thermal shield has conventionally been made from layers of radiative insulation, fiberglass, and coil foil. Coil foil is itself a material of layered construction with a fiberglass cloth support and parallel heat-conducting elements such as copper wires embedded in a film adhesive such as an epoxy. The coil foil is arranged in the thermal shield so that the copper wires extend from the dewar tail upwardly toward the neck of the reservoir that contains the cryogenic fluid. The copper wires conduct the heat received by the thermal shield to the dewar neck for transfer to the evaporated cryogenic fluid.
One problem that has been experienced in the construction of dewars of this type is the difficulty in fabricating the heat shield to the required dimensions, curvatures, and dimensional stability over the broad temperature range experienced by the thermal shield structure. When curvatures are small, it is difficult to form the heat shield in the first place, and there is a tendency for the heat shield to deform and crack in service. Such irregularities in the final heat shield can have a significant adverse effect on the performance of the biomagnetometer. It was sometimes possible to manufacture acceptable dewars of some designs with the prior approach, but success was often dependent upon the skill of the manufacturing personnel.
There is a need for an improved construction for dewars used in magnetometry applications, to permit designers more latitude in selection of the structural designs. Such improved dewars must have good thermal characteristics, and should also be readily fabricated without failures in service. The present invention fulfills this need, and further provides related advantages.